Direct view and projection video display systems sometimes employ automatic kinescope bias (AKB) control systems for maintaining proper black image current levels for each electron gun of an associated image displaying kinescope. Advantageously, displayed picture colors and picture gray scale information are prevented from being adversely affected by variations of kinescope bias from a desired level due to such factors as aging and temperature changes. An example of such a system is described by Tallant in U.S. Pat. No. 4,633,321 entitled AUTOMATIC KINESCOPE BIAS CONTROL SYSTEM which issued Dec. 30, 1986.
A conventional AKB control system includes a beam current sampling element coupled to a control amplifier in an AKB feedback control loop for automatically adjusting the kinescope black level. Generally, this is done by sampling the kinescope drive level during a retract interval (e.g., vertical blanking), comparing the sample with a reference level and applying a correction current to the driver amplifier in a sense to adjust the black level to a desired value.
It is recognized in the above-mentioned Tallant patent that in certain television display systems the addition of an AKB control system may produce undesirable visible artifacts which require correction. One such situation is illustrated in FIGS. 2A and 2B herein which illustrate certain pincushion effects of a displayed raster. FIG. 2A illustrates the geometric distortion of a horizontal line displayed near the top of the television screen. Lines which pass through the center of the screen will be straight but, as shown, lines near the top (or bottom) of the screen will exhibit a curved or arcing shape commonly called the "pincushion" effect. In direct view kinescopes this distortion may be reduced or eliminated by proper yoke design or by means of special pincushion correction signals applied to the yoke. In certain types of kinescopes, however, pincushion correction is applied only during the trace portion of a line and not during the retract portions. An example, discussed by Tallant, is the "planar" kinescope. Certain projection systems also apply pincushion correction to just the active video trace interval and not during the beam retract interval. An effect of not correcting the trace position during the retrace interval is illustrated in FIG. 2B. As seen, the raster line is straight during the trace interval but exhibits a downwardly projecting arc during retrace. Since the beam is normally blanked during retrace, the lack of correction during the retrace interval is not visible for the active lines of the video signal.
A problem arises when AKB control is added to a display system of the type which provides no pincushion correction during retrace intervals. Recall that the AKB measurement current is applied to the kinescope during the vertical interval. Normally, the lines of the vertical interval are off the visible screen. Even though the video is blanked during this period, the injected AKB measurement current will cause beam current conduction during the vertical interval. The corrected lines during the trace interval are not visible since they are all above the top of the visible raster. However, the un-corrected horizontal retrace lines near the end of the vertical interval can arc or "hang" down into the visible portion of the display area.
Stated another way, the un-corrected retrace pattern may be seen in a receiver employing an AKB system of the type which requires kinescope beam current conduction during several line intervals for measurement and adjustment of the beam current. The measurement current is normally applied shortly after the end of the vertical retrace interval during the picture "overscan" portion of the display at the very top of the display screen and so the relatively straight pincushion corrected lines during trace intervals are not seen by the viewer. However the uncorrected retrace pattern exhibits a pronounced downward arcing trajectory at the top of the display screen and some of them may extend into the portion of the display seen by the viewer. The result is that the AKB measurement current can produce an objectionable visible artifact during the unblanked AKB operating interval which may be manifested in the form of several horizontal retrace lines in whole or in part.
In an embodiment of the Tallant apparatus, the problem of objectionable visible artifacts is eliminated by providing a grid drive signal in the form of a composite signal, with positive white-drive pulse components present only during the horizontal trace intervals encompassed by the grid drive signal, and with the grid drive signal being blanked during horizontal retrace intervals. Accordingly, the grid drive signal will induce white-going cathode output current only during the (corrected) trace intervals (FIG. 2B) which are within the kinescope display overscan region and not seen by a viewer.